This study is to evaluate ZDV+3TC versus d4T+ritonavir versus ZDV+3TC+ritonavir with respect to the change in plasma HIV-1 RNA copy number from baseline to 48 weeks in stable HIV-infected children with >= 16 weeks of prior continuous antiretroviral therapy. It will aslo evaluate the safety and tolerance of ZDV+3TC versus d4T+ritonavir versus ZDV+3TC+ritonavir based upon laboratory and clinical toxicities.